Ink jet printers eject ink onto a print medium, such as paper, in controlled patterns of closely spaced dots. FIG. 1 is a schematic view of a typical prior art multi-orifice ink jet print head array 10. Ink is supplied from a reservoir 12 to ink chamber 14A. A piezoceramic transducer (PZT) 16A is bonded to a diaphragm 18A, which constitutes a wall of chamber 14A.
PZT 16A contains electrodes that are connected to a conductor 20A and an electrical ground 22. Signal source 24 applies a voltage signal between conductor 20A and ground 22, thereby creating a voltage difference between the electrodes of PZT 16A. Applying a voltage to PZT 16A causes it to bend and thereby bend diaphragm 18A to change the pressure of the ink in chamber 14A. If the signal has certain well-known waveform characteristics, the diaphragm 18A bends such that the pressure causes an ink drop to be ejected from orifice 28A toward paper 30.
As used herein, the letter "A" following a symbol means that the element identified by the symbol is associated with orifice 28A. Ink drops are also ejected from orifices 28B, 28C, and 28D, which are associated with other respective conductors, PZTs, and chambers, which are not shown but are analogous to conductor 20A, PZT 16A, and chamber 14A.
To print dots on all portions of paper 30, print head array 10 is shuttled back and forth in the X direction, as shown in FIG. 1, as paper 30 is advanced in the Y direction. Because of dot travel time, print head array 10 ejects an ink drop from a particular orifice before it is aligned with the intended destination of the dot. If the velocity of an ink drop is different from what is expected, the ink drop will not strike the intended location on paper 30. The drop location error is emphasized because ink drops can be ejected while the head is traveling in both the positive and negative X directions.
Ideally, each print head will eject ink drops at a predetermined desired velocity. In practice, because of imperfections in manufacturing, there is an unacceptably large deviation between the actual velocities and a desired velocity of the ink drops. Moreover, the speeds of ink drops from some orifices are too high, while the speeds of ink drops from other orifices are too low. As a result of the inaccurate velocities, images printed on paper 30 have certain imperfections such as poorly aligned edges.
As used herein, velocity includes both speed and direction. The speed at which an ink drop is ejected affects both the vertical (e.g., because of gravity) and horizontal (e.g., because of movement of print head 10) position at which the ink drop strikes paper 30. The initial speed also affects the initial direction.
The deviation between the actual velocities and the desired velocity can be reduced by advanced manufacturing techniques. Nevertheless, even with the best known manufacturing techniques and conditions, a print head array will include a high percentage of chambers and orifices that eject ink drops at an unacceptably large deviation from the desired velocity.
There is, therefore, a need for an ink jet print head array in which all of the orifices eject ink drops at velocities within an acceptable velocity range.